In a skeletal muscle, acto-myosin crossbridges, i.e. binding of myosin heads with the thin filaments, are formed under a wide range of conditions, such as in the presence of ATP and Ca++ (contracting state), without Ca++ (relaxed state), in the absence of ATP (rigor state) and in the presence of ATP analogues. Our goal is to characterize the structural, and simultaneously mechanical properties of the crossbridges under these conditions by studying X-ray diffraction patterns and mechanical measurements from single muscle fibers. Low angle equatorial X-ray diffraction patterns have been obtained from the relaxed and rigor rabbit psoas fibers at low and normal ionic strengths and at low temperature. In conjunction with modelling, the results suggested that in the relaxed state, the centers of mass of the myosin heads are not shifted significantly from their detached positions as they become attached. In the rigor state, the tail part of the myosin head moves, probably by a slewing motion, closer towards the thin filament. The structure of the crossbridges in an active muscle could be an intermediate between those found in the relaxed and rigor states. Further work is in progress to obtain high resolution diffraction patterns from actively contracting single muscle fibers. By conducting comparative studies, further understanding of the molecular structure associated with actomyosin interaction could be achieved. In contrast to rabbit psoas muscle, the fin muscle from flatfish showed few crossbridges formed at low ionic strength and 6 degrees C. This probably reflects a difference in kinetic rates in the ATP hydrolysis cycles for the two animals. Secondly, two distinct rigor structures were observed depending on whether the fish muscle was allowed to shorten during the transition into the rigor state. Since the myofilament lattice of the fish is different from those of rabbit muscle and other skeletal muscle, this suggested that the pattern of myosin heads attaching along the thin filaments in a muscle cell depends on the basic lattice structure.